1. Field of the Invention
The present invention relates to an illumination system. More particularly, the present invention relates to an illumination system using a coherent light source.
2. Description of Related Art
Referring to FIG. 1, a conventional laser projection apparatus 100 includes an illumination system 110, a digital micro-mirror device (DMD) 120 and a projection lens 130. The illumination system 110 includes a laser light source 112, a plurality of lenses 114 and a light integration rod 116. The laser beam 113 emitted from the laser light source 112 is expanded by the lenses 114 and uniformed by the light integration rod 116 to become an illumination beam 113a. After being projected onto the DMD, the illumination beam 113a is converted by the DMD into an image beam 113b. The image beam 113b is projected onto a screen (not shown) through the projection lens 130, thus forming a display image.
Because of the high coherence of the laser beam 113, a speckle pattern is generated in the display image on the screen due to the interference phenomenon of the laser beam 113 after the laser beam 113 passes through optical components (such as a lens, a reflector, etc.) having a slightly uneven surface in the laser projection apparatus 100. The speckle pattern is an irregular noise pattern. The speckle phenomenon causes a non-uniform brightness of the display image, thus degrading the display quality of the laser projection apparatus 100.